A voltage source inverter may be used to control the phase currents of a three-phase alternating current (AC) motor, such as incorporated in electric drives. Some voltage source inverters employ a switching circuit (e.g., based on semiconductor switches) to regulate these phase currents and thus, control the fundamental output voltage component of the voltage source inverter. The capacity to activate/deactivate any given switch of the voltage source inverter is typically provided by a gate drive circuit, which provides a gate drive supply voltage to the switch.
Most conventional gate drive circuits are designed with a fixed supply voltage, and this supply voltage is typically selected based on a balance of considerations to account for a number of performance trade-offs. These trade-offs may impact delays, required dead-time, transient response, fault response, or the like. Additionally, the voltage source inverter may be required to operate over a wide temperature range and with wide fluctuations of the main power supply voltage. The supply voltage for the gate drive circuit is often selected such that the voltage source inverter can operate over most, if not all, of these operating conditions and differences resulting from manufacturing tolerances.
In general, the supply voltage for the gate drive circuit is typically maintained at a fixed level below a maximum value for periods of elevated high voltage direct current (DC) supply voltage levels (e.g., elevated DC link voltage). At times, the voltage source inverter may encounter thermal stresses, and during periods of low output electrical frequency, the thermal stresses may not be evenly distributed among the inverter switches. For example, uneven distribution of thermal stresses may occur when the thermal time constant of the inverter is slower than the change in the electrical output angle of the inverter. Typically, this occurs for output electrical fundamental frequencies less than about 3 Hz to a stall condition at about 0 Hz. To thermally protect the inverter switches, the electrical output of the voltage source inverter may be restricted. This generally reduces the current carrying capacity of the inverter. In the past, the silicon die area of the inverter may be increased to increase the current carrying capacity and thereby offset this reduction.
Accordingly, it is desirable to provide methods and apparatus for dynamically controlling a voltage source inverter that reduces conduction losses during low-speed, high-current conditions. In addition, it is desirable to provide methods and apparatus for increasing the current carrying capacity of a voltage source inverter while limiting the losses for a non-varying silicon die area of the inverter. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.